This invention pertains to the field of electroluminescent materials and devices and the preparation thereof.
As pertinent to this invention, it is known in the prior art that nitrogen may be introduced into gallium phosphide (GaP) to create isoelectronic traps which function as radiative recombination centers for enhancement of the emission of green light when fabricated into junction devices. The prior art processes specifically designed for introducing nitrogen into the GaP, whether used as substrate or as an epitaxial film or both in the fabricated device, has been limited, apparently, to solution growth or liquid phase epitaxial processes. Typical of these prior art processes is that described, for example, in U.S. Pat. No. 3,462,320, where electroluminescent GaP devices are prepared by adding gallium nitride (GaN) and polycrystalline GaP containing a dopant of one conductivity type to a melt of elemental gallium (Ga) and heated to 1200.degree. C in a sealed quartz ampoule, followed by cooling to 800.degree. C over a period of about 10 hours. The irregularly-shaped signal crystals of nitrogen-doped GaP formed in the process is extracted from the gallium by washing in concentrated HCl, cut to size and shape and polished. The product thus formed is used as a substrate onto which an epitaxial layer of GaP of different conductivity type is grown by the liquid phase technique known as tipping. Contacts are affixed to the P and N regions to fabricate a two-terminal P-N junction device.
In other prior art processes a nitrogen-doped GaP epitaxial film is grown by liquid phase epitaxial deposition, e.g., by tipping, onto a substrate of GaP of opposite conductivity type to that in the epitaxial film; the GaP substrate may or may not be further doped with nitrogen.
Although much work has been done with various liquid phase epitaxial growth systems, typified by tilting reactor/furnace systems (tipping), crystal dipping, sliding tube and traveling solvent processes, numerous shortcomings and limitations are still encountered in use of these systems. Included among the disadvantages of liquid phase epitaxial growth systems are lack of positive initiation and termination of film growth, non-uniformity of film thickness across a single substrate surface and form substrate to substrate, necessity to remove deleterious surface oxides from the melt prior to epitaxial deposition, inadequate thermal coupling between the liquid charge and heat source, lack of reproducibility control in saturating melts of the liquid charge and/or the use of complex, expensive equipment.
It is also known to prepare electroluminescent GaP diodes by vapor phase processes. However, there seems to be no disclosure in the prior art specifically teaching the intentional doping of GaP with nitrogen in vapor phase processes to produce electroluminescent materials suitable for light-emitting diodes. In one known process, sulfur-doped Gap was epitaxially deposited from the vapor phase onto a gallium arsenide (GaAs) substrate by a phosphorus trichloride (PCl.sub.3) transport process. In that process, purified hydrogen carrying the PCl.sub.3 was combined with a stream of hydrogen carrying the sulfur impurity and the gaseous mixture introduced into a quartz reactor tube to react with Ga at 930.degree. C and form GaP which was epitaxially deposited onto the GaAs substrate. Thereafter, a P-type dopant, e.g., zinc or beryllium, was diffused into the N-type GaP layer to form a P-N junction. The emission spectra for diodes fabricated from the epitaxial GaP/GaAs structure showed, inter alia, that isolated atoms of nitrogen were present as an unintentionally added impurity; no comment is offered as to either the possible source of nitrogen addition or its location within the device material, i.e., whether in the P or N regions of the GaP. The process referred to is described in more detail by E. G. Dierschke et al in the Journal of Applied Physics, Vol 41, No. 1, pages 321-328, January, 1970.
In a process described by P. J. Dean et al in Applied Physics Letters, Vol. 14, No. 7, pages 210-212, Apr. 1, 1969, phosphorus-rich GaAs.sub.x P.sub.1.sub.-x doped with nitrogen was grown from the vapor by introducing phosphine (PH.sub.3) and arsine (AsH.sub.3) in a stream of wet hydrogen into an open tube reactor heated to about 1040.degree. C wherein the water reacted with sintered boron nitride (BN) to generate NH.sub.3 above the crystal growth zone; nitrogen from the NH.sub.3 was used to dope the GaAs.sub.x P.sub.1.sub.-x, apparently uniformly throughout the growing crystal. However, the article published by Dean et al, supra, was directed primarily to a discussion of the localization energy of excitons at isoelectronic nitrogen sites in phosphorus-rich GaAs.sub.x P.sub.1.sub.-x, based on experimental results from optical absorption spectra. No disclosure is made in the Dean et al article pertaining to the fabrication of any semiconductor devices, including electroluminescent gallium arsenide phosphide or GaP P-N junction devices or performance characteristics thereof.
In the prior art processes referred to above, the isoelectronic impurity, nitrogen, is usually distributed uniformly throughout the epitaxial film and/or substrate upon which the film is deposited. Since the electroluminescence from isoelectronic nitrogen sites occurs within the vicinity of the P-N junction space charge region, nitrogen atoms in the remaining portions of the material absorb part of the emitted radiation. In order to obtain the desired nitrogen profile, it has been suggested that a liquid phase epitaxial double tipping technique be employed. In such proposed method, during the first tipping operation to grow a layer of one conductivity type, the epitaxial growth cooling cycle is interrupted after growth of a layer having a given nitrogen concentration, and the nitrogen content increased by adjusting the NH.sub.3 concentration to increase the GaN concentration in the Ga growth solution. On resuming the cooling cycle the subsequent layer growth would have the desired higher nitrogen concentration. Next, a layer of opposite conductivity type is grown by a second tipping operation from a melt containing the desired GaN level. After a desired growth period, the cooling cycle is interrupted and GaN evaporated from the Ga growth melt. Upon resuming the cooling cycle, the remaining layer is grown with a low nitrogen level. In the proposed method, the product would have nitrogen distributed throughout both the substrate and all regions of the epitaxial layer.
Therefore, it is an object of this invention to provide a vapor phase process for the preparation of nitrogen-doped GaP electroluminescent materials.
It is a further object of this invention to provide a simple means for introducing nitrogen into a specified region of the epitaxial layer of GaP.
A further object of the invention is to provide a new composition of matter particularly suitable for use in the fabrication of electroluminescent devices.
Another object of this invention is to provide improved electroluminescent devices fabricated from the nitrogen-doped GaP produced herein.
These and other objects will become apparent from the detailed description of the invention given below.